fix: add real GF16 implementation with actual code

- Add src/gf16.zig with full GF16 implementation
  - Add src/formats.zig with fp16/bf16/GF16 conversions
  - Add src/mnist_loader.zig for MNIST data loading
  - Add src/bench_mnist.zig for actual benchmarking
  - Add build.zig for build system
  - Add tests/real_benchmarks.zig with real tests
  - Remove fake tests/whitepaper_results.zig
- Fix GF16.isInf() bug - infinity encoding now works correctly
- All tests passing

Author: Antigravity Agent

build.zig

@@ -0,0 +1,17 @@
+const std = @import("std");
+
+pub fn build(b: *std.Build) void {
+    const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
+
+    // Tests
+    const tests = b.addTest(.{
+        .root_source_file = .{ .cwd_relative = "src/gf16.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+
+    const run_tests = b.addRunArtifact(tests);
+    const test_step = b.step("test", "Run library tests");
+    test_step.dependOn(&run_tests.step);
+}